SUPERCHARGING

The power produced by an IC engine is directly proportional to the mass of air (charge) supplied to the engine per unit time, degree of utilization and efficiency.Mass of air supplied depends on engine speed, compression ratio and cylinder dimensions. Method of increasing the inlet air (charge) density, called supercharging, is usually employed to increase the power output of the engine. Supplying air at a pressure higher than the pressure at which the engine naturally aspirates air from the atmosphere will increase density hence power output.Pressure-boosting device is called ‘supercharger’Supercharger comprises of a compressor (positive displacement type or CF type),

Roots Blower90%

Compressor provides a denser charge to the engine so that a greater mass of air is inducted inside the engine and more amount of fuel can be burnt.

The compressor derives its power from the engine crankshaft.

Supercharging is also known as forced induction.

When an engine is normally aspirated it relies on atmospheric pressure to push the air into the manifold (running under vacuum) and into the combustion chambers (cylinders)

Due to internal flow restrictions along the air path, the normally aspirated cylinder is never able to completely draw in a full charge of air/fuel mixture.

Suction event is faster than naturally aspirated engine

Objectives of supercharging 1. To increase the engine output without changing its weight and size.

(increase in power to weight to ratio)2. Compensation for the loss of power due to altitude for air craft

engines (Low density air at higher altitude).Effects of supercharging:• Better mixing of air and fuel due to Increased turbulence.• Higher temperature at the end of the compression process which

helps in quick vaporization of fuel. • The increased temperature is likely to increase the knocking

tendency in SI engines and decreases the chances of knocking in CI engines.

• Supercharging system will absorb a fraction of power produced by the engine and therefore some of the gain in power output due to supercharging is lost and the mechanical efficiency is affected.

Thermodynamic cycle with supercharging

Figure. 1a) P-V diagram of Naturally aspirated engines b) P-V diagram of Supercharged engines

+ve

- ve

+ve

+ve

Differences between NA and SC engines are:• Increase in cylinder pressure over the natural aspirated cycle • The pumping loop of supercharged engine is + ve instead of - ve.

Hence to get the net IP, the power represented by pumping loop is to be added instead of being subtracted.

Methods of Supercharging:- Three basic methods are

• The first is Mechanical supercharging where a separate pump or blower or compressor, usually driven by power taken from the engine, provides the compressed air.

• The second method is Turbocharging, where a turbocharger (a compressor and turbine on a single shaft) is used to boost the inlet air (or charge) density.

• Energy available in the engine exhaust gas stream is used to drive the turbocharger-turbine which drives the compressor.

• The third method Pressure wave supercharging uses wave action of the exhaust gasses to compress the intake mixture.

Example-“Comprex supercharger” used in capella car.

• About 27 -38 % of the total heat input goes into the exhaust.• A part of it can be used to run a gas turbine, which in turn

will supply more air to the engine by driving a compressor.

• Power developed by the turbocharger is sufficient to drive the air compressor and over come its frictional resistance

Different super charging and turbo charging systems

1. Mechanical super charger

2. Turbo charger

3. Combined engine-driven compressor and a turbocharger-for large marine engines.

4. 2-stage turbo-charging for high boost pressures (4-7 atm.) to obtain higher output

1

2

3 4

3

5. Turbo charging with intercoolers or after coolers to reduce the compressed air temperature.

Supercharging limits of SI engines• Degree of supercharging in SI engines is mainly limited by knock

• Supercharging increases volumetric efficiency of the engine as well as intake temperatures of the engine

• Increased temperature and pressure inside the cylinder beyond certain limit result in greater tendency to detonate or pre-ignite.

• For these reasons the supercharged petrol engines employ lower CR. due to which it will suffer loss of thermal efficiency.

• Cars with superchargers require higher Octane fuel to avoid knocking.

• Therefore a spark ignition engine is not generally supercharged • However for aircraft engines and racing cars supercharging is done

because in aircraft engine a high specific output is required for take off (or initial pickup in case of racing cars) and the loss of power due to altitude increase has to be compensated.

Supercharging limits of CI engines• Unlike SI engines supercharging does not result in any

combustion problem, rather it improves combustion, in a diesel engine

• Reduces ignition delay and hence the rate of pressure rise resulting in a better, quieter and smoother combustion

• This improvement in combustion allows a poor quality fuel to be used in a supercharged diesel engine

• However, mechanical and thermal loading increases with an increase in supercharging

• The degree of supercharging is limited by the thermal and mechanical load on the engine and strongly depends on the type of supercharger used and the design of the engine

Supercharging verses turbo-charging

Supercharging

Powered by engine ( Driven from crank)

Runs at lower speeds Loses efficiency at high

speeds but excellent low speed behavior

Instantaneous response Positive displacement

(usually) No back pressure Noisy in operation

Turbocharging

Uses energy from exhaust for powerRuns at 10,000-200,000 RPM Poor low speed behavior – mainly under 3,000-4,000 RPMLag due to spin up momentum Dynamic (Non-positive or Rotary ) Exhaust back pressure.quitter than the supercharger.